Field of the Invention
The present invention relates to a turbine engine casing, in particular for a turbine engine such as an airplane turboprop or turbojet.
Description of the Related Art
A fan casing typically comprises a substantially cylindrical wall that extends around the fan blades of the turbine engine and that has an inside surface covered in acoustic insulation panels. In general, these panels comprise an annular honeycomb structure with inside and outside faces that are each covered in a skin that may be multiply perforated in order to improve the treatment of sound. They are for absorbing the soundwaves that are generated by the fan of the turbine engine.
The following prior applications in the name of the Applicant EP 2 318 679, EP 2 088 290, and FR 2 965 859, describe acoustic insulation panels of that type.
An acoustic insulation panel of a turbine engine is generally sectorized, i.e. made up of a plurality of panel sectors arranged circumferentially end to end and secured to the wall of the casing by adhesive or by fastener screws that extend radially relative to the longitudinal axis of the turbine engine. Fastening sectors by means of screws presents drawbacks since a large number of fastener screws need to be used, thereby firstly increasing the weight of the turbine engine, and secondly running the risk of damaging the fan blades in the event of loss or breakage. Furthermore, the interface zones of greater or smaller width between the sectors give rise to an alternation of smooth zones and of treated zones in the vicinity of the fan (thereby creating discontinuities in acoustic impedance), leading to an increase in the noise levels from the turbine engine at certain operating speeds. Furthermore, mounting numerous fastener screws to the panels requires them to be provided with densified zones, thereby degrading acoustic efficiency.
In order to limit the number of fastener screws, proposals are made in patent application FR 2 935 017 to lengthen the fan casing and to fasten a single-piece annular panel for acoustic insulation to the wall of the casing and to the air inlet sleeve. In practice, such an acoustic panel is difficult to maintain, given that it is necessary to take the turbine engine out of service in order to replace a damaged panel with a new panel.
Furthermore, a casing wall may present deformations and may have an inside surface that is not accurately cylindrical. This applies in particular to a casing wall made of composite material that, because of the way in which it is fabricated, can present dimensional tolerances that are relatively large. In the prior art, the acoustic panels that are fastened to a casing wall of that type do not enable the above-mentioned tolerances to be compensated.
That applies in particular to acoustic insulation panels that are adhesively bonded to the wall of the casing by means of a thermosetting adhesive. Such adhesive bonding takes a long time to achieve (several hours in an autoclave of large dimensions) and, in the event of the panel being damaged, it is necessary to remove the engine in order to replace the panel.
In order to remedy that drawback, patent application FR 12/60493 in the name of the Applicant proposes a turbine engine casing having a substantially cylindrical wall and a single-piece annular panel for acoustic insulation mounted inside the wall. Elastically deformable tabs are fastened to the panel, or respectively to the wall, and they include orifices for receiving the ends of screws fastened to the casing, or respectively to the panel.
Such a casing thus has a fastener system enabling the panel to be removed easily, in particular under the wing of an airplane, i.e. without removing the engine.
Given the resilience of the tabs, it is nevertheless difficult to ensure that the panel is accurately positioned inside the wall. Such tabs may also deform or crack under the effect of the mechanical or thermal stresses that are applied to them in operation.
Patent application FR 12/60495 in the name of the Applicant discloses a turbine engine casing comprising a substantially cylindrical wall and a single-piece annular panel for acoustic insulation mounted radially inside the wall, the panel including a radially outer annular surface fitted with upstream and downstream first projecting members respectively arranged in the upstream and downstream portions of the radially outer annular surface of the panel, the wall including a radially inner annular surface fitted with upstream and downstream second projecting members that are releasably fastened respectively to the upstream and downstream first projecting members. By way of example, the projecting members are formed by tabs fastened to the panel and to the wall of the casing, and the releasable fastening is provided by nut-and-bolt systems.
Such a method of fastening the single-piece panel to the wall of the casing is statically indeterminate and does not enable the panel to be positioned or centered accurately inside of the wall of the casing. Specifically, under such circumstances, positioning or centering is provided by the above-mentioned bolts engaged with clearance in the corresponding holes of the projecting members.